Polymer gels possess a number of interesting properties which solids or liquids cannot singly have. However, when an attempt is made to utilize such gels for consumer applications or in the medical and industrial fields, their primary limitation in performance is often inadequate mechanical strength. This is particularly true in tissue engineering applications where cells must be encapsulated in a hydrogel prior to implantation in the body in the desired site to regenerate damaged or diseased tissues.
Recently, research has involved so-called “dual network” structures potentially arranged as either interpenetrating networks (“IPNs”) or as semi-IPNs. Recent work by Gong, Osada, and coworkers illustrated the potential of such polymer systems tested under compression. See Gong et al., Double-Network Hydrogels with Extremely High Mechanical Strength, Advanced Materials 15(14) 1155-1158 (2003); Na et al., Structural Characteristics of Double Network Gels with Extremely High Mechanical Strength, Macromolecules 37(14) 5370-5374 (2004); Osada et al., U.S. Patent Application No. 2005/0147685 titled “Hydrogel of (Semi) Interpenetrating Network Structure and Process for Producing the Same”; Gong et al., U.S. Patent Application No. 2006/0003442 titled “Cell Culture Scaffold, Methods of Manufacturing the Scaffold and Cell Culture Methods”; Osada et al., U.S. Patent Application No. 2004/0116305 titled “Low Friction Hydrogel Having Straight Chain Polymers and Method for Preparation Thereof.” In addition, the present inventors investigated PAMPS/PAAm, poly(2-acrylamido-2-methylpropanesulfonic acid) (“PAMPS”)/polyacrylamide (“PAAm”), and agarose/poly(2-hydroxyethyl methacrylate) (“PHEMA”) as two IPN and semi-IPN gels as synthetic model analogs to insect cuticle. In those experiments, the agarose/PHEMA network gel was irradiated by UV light for about 6 to 24 hours which yielded a maximum fracture stress of 0.65±0.5 MPa with a fracture strain of 201±33% at a water content of approximately 79%. See Eichler et al., Abstract, Insect Cuticle as a Motif for Biomimetic Materials, in Proceedings of the 2005 AIChE Annual Meeting (Oct. 31, 2005).
The present invention is directed to an improved hydrogel network, and in particular one that is well adapted to maintain viability of mammalian cells encapsulated therein.